Recent developments in semiconductor technology include the complementary metal-oxide-semiconductor (CMOS). CMOS is a technology employed in constructing integrated circuits that yields semiconductor devices having a wide variety of uses in electronic components. These uses can include, for instance, microprocessors, microcontrollers, random access memory, storage memory, and other digital logic circuits. Analog uses for CMOS technology include data integrators, and integrated transceivers employed in electronic communication, as well as image sensors for cameras and video recording devices.
One particular type of image sensor leveraging CMOS technology is the CMOS image sensor. Like many CMOS devices, CMOS image sensors are flexible devices that can be integrated with other analog or digital components, or can be incorporated into a System-on-Chip (SoC). In the latter form, an image sensor can be integrated with various other components (e.g., analog, digital, . . . ) associated with capturing or processing an image, on a common integrated circuit chip. For instance, the SoC can comprise a single piece of silicon that includes the image sensor, a microprocessor, microcontroller, or digital signal processor (DSP) core, memory, analog interfaces (e.g., analog to digital converters, digital to analog converters), among other components.
Imaging devices utilizing CMOS imaging sensors can have reduced manufacturing costs, reduced power consumption, or reduced electronic noise, while achieving higher optical resolutions. For instance, cameras can use CMOS imaging System-on-Chip (iSoC) sensors that efficiently marry low-noise image detection with sophisticated signal processing, in conjunction with supporting blocks that can provide timing control, clock drivers, reference voltages, analog to digital conversion, digital to analog conversion and image processing elements. High-performance video cameras can thereby be assembled using a single CMOS integrated circuit supported by few components including a lens and a battery, for instance. Accordingly, by leveraging iSoC sensors, camera size can be decreased and battery life can be increased. The iSoC sensor has also facilitated the advent of more advanced optical recording devices, including dual-use cameras that can alternately produce high-resolution still images or high definition (HD) video.
An image sensor converts an optical image into an electronic signal, which can then be processed and reproduced, for instance on a display screen. Typically, the image sensor comprises an array of many active pixels; each active pixel comprises a CMOS photodetector (e.g., photogate, photoconductor, photodiode, . . . ) controlled by circuits of digitally managed transistors. The CMOS photodetector can absorb electromagnetic radiation in or around the visible spectrum (or more typically a subset of the visible spectrum—such as blue wavelengths, red wavelengths, green wavelengths, etc.), and output an electronic signal proportionate to the electromagnetic energy absorbed.
Electronic imaging devices, such as digital cameras and particularly video recorders, capture and display many optical images per second (e.g., 30 per second, 60 per second, 70 per second, 120 per second, . . . ), equal to the optical frame rate of the imaging device. Capturing a single image in a single frame time involves multiple operations at the CMOS pixel array and readout circuit. One mechanism for image capture is referred to as a rolling shutter. As an example, rolling shutter operations can include capture and convert (e.g., capture light information and convert to electrical information), readout, and reset operations. Some frames can be constructed so that the capture and convert operation, and the reset operation are performed in a single reset cycle, for instance, with reset of a prior frame occurring at a beginning of the reset operation, and capture and convert of a current frame occurring at the end of the reset operation. Thus, alternating reset and readout cycles can clear the CMOS photodetector array, capture a new image, and output the captured image for processing.
Another mechanism for controlling electronic shutter operations for a CMOS image sensor is a global shutter operation. For global shutter operations, all pixels of the CMOS image sensor are reset concurrently. This particular reset is referred to as a global reset. After being reset, the pixels are configured to collect light for an exposure period, (typically having a predetermined duration). Charge is transferred from photodiodes of respective pixels to a floating diffusion node; the transfer of charge can again be performed concurrently for all pixels, referred to as a global transfer.
For certain global shutter pixels, a correlated double sampling (CDS) operation is conducted over multiple frames. To achieve CDS, a floating diffusion node that will ultimately be used to store the signal charge from the photodetector is reset, and a reset voltage is read out (or output) with respect to a reference voltage. This readout is referred to as a “reset frame”. A global transfer is performed to transfer charge from the image sensor's photodetectors to a corresponding floating diffusion node of the image sensor. Voltage on the floating diffusion node is read out, again with respect to the same reference voltage. This readout is referred to as a “readout frame”. The reset frame subtracted from the readout frame provides the actual image value sans the correlated noise present in each of the two frames. CDS can also be performed entirely within the sensor if there is means for storing the reset frame and subsequently subtracting it from the signal frame.
The global shutter can provide advantages over the rolling shutter operation. For instance, global shutter mitigates or avoids some undesirable artifacts observed in the rolling shutter operation, like geometric deformation of moving objects stemming from the capture of object movement at a rate faster than the frame capture rate. Additionally, global shutter operations need not employ a mechanical shutter to capture still images. However, global shutter does present some challenges, which largely are the focus of ongoing research and development in CMOS image sensor technology.